The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. Such scaling down has also increased the complexity of processing and manufacturing ICs.
In ICs, semiconductor devices are formed on semiconductor substrates, and are connected through metallization layers. The metallization layers are connected to the semiconductor devices through contact plugs. Also, external pads are connected to the semiconductor devices through the contact plugs.